1. Field of the Invention
The present invention relates to a control apparatus of a machine tool including a feed shaft motor for driving a feed shaft and a main spindle motor for driving a main spindle and in particular relates to the control apparatus of the machine tool in which AC supplied from an AC power supply side is converted to output DC which is again converted to AC for driving the motors, which is supplied to drive the feed shaft motor and the main spindle motor.
2. Description of the Related Art
In a machine tool including a feed shaft motor for driving a feed shaft and a main spindle motor for driving a main spindle, DC power temporarily converted from AC power inputted from an AC input side is again converted to AC power and the AC power is used as driving power for motors mounted on each driving shaft. Such a machine tool comprises: as a main circuit of a motor control apparatus, a converter for outputting a DC voltage by rectifying an AC voltage supplied from an AC power supply side including a three-phase AC input power supply; and an inverter, which is connected to a DC link (direct current link) being a DC side of the converter, for performing a bi-directional power conversion between DC power of the DC link and AC power which is driving power for the motors or regenerative power, and the machine tool controls speed, torque, or a position of a rotor of a main spindle motor and a feed shaft motor connected to an AC output side of the inverter.
The inverter among them is provided for each motor driving each of a plurality of drive shafts.
In recent years, in response to demands for energy conservation, inverters capable of regenerating power are often used for reusing, as driving power of motors, regenerative power generated during deceleration of the motors by storing it in a storage device provided in the DC link and for further restoring the regenerative power to the AC power supply side.
On the other hand, with regard to the converter, one converter corresponding to a plurality of inverters is often provided in order to reduce the cost of and the space occupied by the motor control apparatus in the machine tool. Furthermore, similarly to the aforementioned inverters, there are also cases where converters capable of regenerating power are used in response to demands for energy conservation, the converters being capable of restoring regenerative energy generated during deceleration of motors to the AC power supply side.
When a power outage occurs at the AC power supply side, the aforementioned motor control apparatus becomes unable to continue the normal operation of the feed shaft motor and the main spindle motor. In such a case, it is required to stop the operation of the feed shaft motor driving a feed shaft as soon as possible to prevent a collision of the feed shaft. For this reason, when a power outage occurs at the AC power supply side, it is implemented that a tool connected to the feed shaft motor, a workpiece machined by the tool, and the like are protected by issuing a deceleration command to the feed shaft motor to stop in conjunction with a detection of a power outage. In a case that the power supply of a control apparatus (a computer unit) is backed up by an uninterruptible power supply (UPS), the control apparatus can instruct a feed shaft motor inverter action to be taken in case of emergency even when a power outage occurs at the AC power supply side, and it can be controlled in such a way that the feed shaft motor inverter is operable for a while with charges stored in capacitors provided in a converter (a rectifier) and the feed shaft motor is urgently stopped.
For example, a method in which an early halt of a motor is realized by actively generating reverse torque during deceleration of the motor as described in Japanese Laid-open Patent Publication No. H07-143780 has been proposed as a method of urgently halting the motor upon a power outage at the AC power supply side.
However, if a technique as described in Japanese Laid-open Patent Publication No. H07-143780 and the like, for example, in which an emergency halt is carried out by providing a deceleration command to a feed shaft motor in conjunction with a detection of a power outage at the AC power supply side, was applied to a motor drive apparatus having a function of regenerating to the AC power supply side regenerative power generated during deceleration of the motor, the regenerative power could not be restored to the AC power supply side during a power outage and, hence, a DC voltage at a DC link between a converter and an inverter would increase. In particular, it is significant when the regenerative power of a motor is large. Usually, an inverter issues an “overvoltage alarm” to protect the inverter itself and aborts control when the DC voltage at the DC link being the DC side thereof increases excessively. In this case, an emergency halt such as halting the motor by actively generating reverse torque during deceleration is difficult to be performed, and hence a problem arises that it takes time from an occurrence of a power outage at the AC power supply side till a halt of the motor. If such a problem arose, for example, in the aforementioned case of the feed shaft motor, a collision of a feed shaft could not be prevented.
Depending on characteristics of a feed shaft motor and a situation of frictions received by a feed shaft driven by the feed shaft motor, it may be required to continue supplying drive power from a feed shaft motor inverter to the feed shaft motor even when the feed shaft motor is to be decelerated. In other words, in this case, since regenerative power is not generated in the feed shaft motor even during deceleration of the feed shaft motor, the feed shaft motor inverter does not provide energy to the DC link, but conversely the feed shaft motor inverter converts DC power of the DC link to AC power to supply to the feed shaft motor. In this situation, when a deceleration command is provided for an emergency halt as described above upon an occurrence of a power outage at the AC power supply side, the DC voltage of the DC link drops rapidly. Usually, when the DC voltage at the DC link being the DC side thereof drops excessively, an inverter is difficult to supply driving power, therefore the inverter issues a “low voltage alarm” and aborts control. In this case, an emergency halt such as halting the motor by actively generating reverse torque during deceleration is difficult be performed, and hence a problem arises that it takes time from an occurrence of a power outage at the AC power supply side till a halt of the motor. If such a problem arose, for example, in the aforementioned case of the feed shaft motor, a collision of the feed shaft could not be prevented.